1. Field of Invention
The present invention relates to phase locking technology. More particularly, the present invention relates to a phase locking system for a three-phase alternating current (AC) electric grid and a phase locking method thereof.
2. Description of Related Art
Currently, along with the development of science and technology and greatly rich in material life, the problem of energy and environment is becoming more and more serious. Consequently, exploitation and utilization of a renewable energy has become a top research and development direction in solving the above-mentioned problems. For example, a wind power generation is worthy of being one of plural renewable energies with its characteristics, such as clean and environment protection.
An existing wind power system mainly adopts a doubly fed induction generator (DFIG). A back-to-back double pulse-width modulated voltage source inverter is used as a primary control system of DFIG, and the inverter includes a grid-side converter and a rotor-side converter (or referred to as a generator-side converter). Under normal operating conditions, only a fundamental forward voltage exists in the electric grid, a voltage synchronizing signal, as a control basis, may be easily obtained by adopting a conventional voltage zero crossing detection or a phase locked loop (PLL) technique. However, in an actual electric grid, centralized application of devices such as a grid-connected inverter, an active filter and reactive compensation equipment in the wind power system often leads to asymmetric failures such as an unbalanced load or single-phase and bi-phase short circuits to earth, causing an asymmetric electric grid voltage.
Particularly, for the DFIG wind power system, on one hand, due to the asymmetric electric grid voltage, a stator voltage and a stator current also include a negative sequence component in addition to a positive sequence component, but a conventional proportional-integral current regulator cannot control the positive sequence component and the negative sequence component simultaneously, and thus a very small unbalance of the stator voltage will lead to a very large unbalance of the stator current, an electromagnetic torque and active power fluctuation, thereby leading to deterioration of the DFIG operation performance. On the other hand, when various current transformers of the DFIG are controlled, it is often needed to use a phase and amplitude of a voltage AC variable, and thus the phase locking technology is one of the key links. When the electric grid is unbalanced due to a failure in the three-phase electric grid, the electric grid voltage includes a positive sequence component and a negative sequence component, and in order to obtain phase information of the electric grid voltage accurately, the negative sequence component should be separated out from the electric grid voltage, so as to obtain a pure positive sequence component and thus phase lock based on the positive sequence component. However in an existing phase locking circuit design, the above-mentioned negative sequence component cannot be separated out completely, and a necessary separation measure is not adopted for a disturbance variable included in the electric grid voltage when the electric grid is unbalanced.
In view of this, it is a subject demanding prompt solution from relevant skill members in the art how to design a phase locking system for the three-phase AC electric grid, so as to separate out the disturbance variable in a voltage signal actively through the phase locking system, and thus obtain a pure fundamental positive sequence component for phase locking accurately.